New default behavior for scroll anchoring (rolled out in Chrome 84?) leads to unsightly/odd accordion interactions - see #31341
This rule suppresses this new behavior and reverts back to the old way.

See https://drafts.csswg.org/css-scroll-anchoring/

Allows use of background-color and more. Fixes #29033 for v5.

Fix card list group borders & radii

- Rename and move the variable to variables file
- Move code to the grid file
- Use the mixin to generate our own classes
- Wrap in a grid classes enabled conditional
- Document the mixin

Trying to find a new way to do responsive card decks while not locking ourselves into the cards themselves. My thinking here is we can easily control the column (.col) width by the parent, but I don't know how many we need (have 0-5 now) across each breakpoint. This works for cards so far, and I think could get us equal height, too.

Define new variable for card header color

Uses orphans/widows trick from @fran-worley at https://github.com/twbs/bootstrap/issues/20925#issuecomment-333492739.

Fixes #20925.

1. Instead of having two selectors, we use the `.list-group-item` class and have coverage for list items and anchors.
2. Rather than override all border values, we only override what we set with `border-top`.